I'm using the GPS-18. Works pretty good, except inside my truck. In my last vehicle and my car, I had no problem sticking it on the dash. With the new truck ('11 dodge ram diesel), it works fine until I start moving, then it loses satellites. Had to mount it outside on the roof. The roof mount isn't a problem (the puck is weathertite), just had to figure out how to get the wiring done.
Anyway, I had a Deluo gps that I liked really well. It was a bit smaller than the gps-18 and it had a 6 or 8 pin connector (similar to a keyboard connector) between the db-9 and the puck. That made it a lot easier to install and made for smaller holes to find/make. Unfortunately, they have been discontinued. Sadly, my deluo didn't survive removal from my last car. The cable got stuck and pulled out.
Oh, I haven't hooked it up to my 396xt, yet. But I believe it requires a null modem adapter and a gender changer. Not totally sure about that, though.
Jeff.